The tiny amplifier that could supercharge quantum computing

Quantum computers are built to handle problems that are far too complex for today’s machines. They could lead to major advances in areas like drug development, encryption, AI, and logistics.

Photo by Chalmers University of Technology

Now, researchers at Chalmers University of Technology in Sweden have developed a new type of amplifier that only switches on when it’s reading data from qubits. Because of its smart design, it uses just one-tenth the power of the best amplifiers currently available. This helps reduce interference with the qubits and could make it possible to build larger, more powerful quantum computers.

Traditional computers use bits, which can be either 0 or 1. Quantum computers use qubits, which can be 0, 1, or any combination of both at the same time. This is known as superposition. It allows a quantum computer with 20 qubits to represent over a million different states at once. Superposition is one of the key reasons quantum computers can tackle problems that even the most powerful supercomputers can’t solve.

Amplifiers are necessary but they create problems

To make use of a quantum computer, you have to measure qubits and turn their signals into something readable. This requires extremely sensitive microwave amplifiers to pick up the weak signals. But reading quantum data is tricky. Small changes in temperature, background noise, or electromagnetic interference can disrupt the qubits and erase the data. Amplifiers add to the problem because they produce heat, which can also cause qubits to lose their quantum state. This process is called decoherence. That’s why researchers are always looking for ways to build amplifiers that use less power and generate less heat.

Now, a team at Chalmers University has made progress with a new, much more efficient amplifier.

“This is the most sensitive amplifier that can be built today using transistors. We’ve now managed to reduce its power consumption to just one-tenth of that required by today’s best amplifiers – without compromising performance. We hope and believe that this breakthrough will enable more accurate readout of qubits in the future,” says Yin Zeng, a doctoral student in terahertz and millimetre wave technology at Chalmers, and the first author of the “Pulsed HEMT LNA Operation for Qubit Readout” study.

A key step toward building larger quantum computers

This new amplifier could play an important role in making quantum computers bigger, with many more qubits than we have now. Chalmers has been working on this challenge for years through the Wallenberg Centre for Quantum Technology, a national research program.

The more qubits a quantum computer has, the more powerful it becomes and the better it can handle complex problems. But scaling up also means adding more amplifiers, which increases power use. That extra power can generate heat and noise, making it harder to keep the qubits stable.

“This study offers a solution in future upscaling of quantum computers where the heat generated by these qubit amplifiers poses a major limiting factor,” says Jan Grahn, professor of microwave electronics at Chalmers and Yin Zeng’s principal supervisor.

Activated only when needed

Unlike other low-noise amplifiers, the new amplifier developed by the Chalmers researchers is pulse-operated, meaning that it is activated only when needed for qubit amplification rather than being always switched on.

“This is the first demonstration of low-noise semiconductor amplifiers for quantum readout in pulsed operation that does not affect performance and with drastically reduced power consumption compared to the current state of the art,” says Jan Grahn.

Since quantum information is transmitted in pulses, one of the key challenges was to ensure that the amplifier was activated rapidly enough to keep pace with the qubit readout. The Chalmers team addressed this by designing a smart amplifier using an algorithm that improves the operation of the amplifier. To validate their approach, they also developed a novel technique for measuring the noise and amplification of a pulse-operated low-noise microwave amplifier.

“We used genetic programming to enable smart control of the amplifier. As a result, it responded much faster to the incoming qubit pulse, in just 35 nanoseconds,” says Yin Zeng.